Unlimited reader stack growth with SERD_NQUADS

Hi, when in n-quads parsing mode the reader grows its internal stack indefinitely.

Version: 0.32.6

How to reproduce

1. Place printf("Push: %fGiB\n", stack->buf_size / 1024.0 / 1024.0 / 1024.0);
   in this branch https://gitlab.com/drobilla/serd/-/blob/main/src/stack.h?ref_type=heads#L54

2. Run

   // The only really relevant thing is SERD_NQUADS
   SerdReader *rdr = serd_reader_new(SERD_NQUADS, NULL, NULL, NULL, NULL, NULL, NULL);
   // tested with yago4english-no-literals-hermit-inf.nt (~40Mio triples)
   serd_reader_read_file(rdr, (uint8_t const *) "large-ntriples-file.nt"); 
   serd_reader_free(rdr);

3. Example Output

   Push: 0.000006GiB
   [..]
   Push: 1.645810GiB
   Push: 2.468715GiB
   Push: 3.703072GiB
   Push: 5.554609GiB
   Push: 8.331913GiB

Expected Behaviour

In SERD_NTRIPLES mode the stack never grows beyond the initial 4096byte allocation.